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Vulnerability record · CVE-2023-43622 · published 23 October 2023

CVE-2023-43622: Apache HTTP Server HTTP/2 zero-window connection denial of service

Apache · Http Server

Apache HTTP Server versions 2.4.55 through 2.4.57 fail to terminate an HTTP/2 connection that opens with an initial window size of 0, allowing the connection to be held indefinitely. This lets an unauthenticated remote attacker tie up worker resources in a slow-loris style pattern. It is fixed in 2.4.58, which terminates such connections after the configured connection timeout.

7.5 CVSS 3.1 High EPSS 71% · top 0.6% CWE-400 · Uncontrolled resource consumption
7.5CVSS 3.1 base score
71%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
4References
17 Jun 2026Last modified by NVD

Description

An attacker, opening a HTTP/2 connection with an initial window size of 0, was able to block handling of that connection indefinitely in Apache HTTP Server. This could be used to exhaust worker resources in the server, similar to the well known "slow loris" attack pattern. This has been fixed in version 2.4.58, so that such connection are terminated properly after the configured connection timeout. This issue affects Apache HTTP Server: from 2.4.55 through 2.4.57. Users are recommended to upgrade to version 2.4.58, which fixes the issue.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 19 September 2026. Confidence: high.

high priorityRemote, unauthenticated, low-complexity availability impact on a widely deployed server, with high EPSS despite no KEV listing or confirmed public exploit.

What it is

Apache HTTP Server versions 2.4.55 through 2.4.57 fail to terminate an HTTP/2 connection that opens with an initial window size of 0, allowing the connection to be held indefinitely. This lets an unauthenticated remote attacker tie up worker resources in a slow-loris style pattern. It is fixed in 2.4.58, which terminates such connections after the configured connection timeout.

Impact

An attacker can exhaust server worker resources, degrading or denying service to legitimate clients. There is no confidentiality or integrity impact; only availability is affected.

Attack surface

Reachable over the network via HTTP/2 on the affected server; no authentication and no user interaction are required per the CVSS vector (AV:N/AC:L/PR:N/UI:N). The attacker only needs to open a connection with an initial window size of 0 and keep it open.

Exploitation

Not listed in CISA KEV and no ransomware usage is documented. EPSS is high (about 0.706, 99.4th percentile), and references are only vendor and third-party advisories, so no public exploit code is confirmed by this record.

What to do

  • Upgrade Apache HTTP Server to 2.4.58 or later, which fixes the issue.
  • If immediate upgrade is not possible, disable HTTP/2 (e.g., remove or comment out Protocols h2/h2c) until patching is complete.
  • Tune connection and request timeouts and worker/thread limits to reduce the impact of held connections.
  • Monitor worker/thread pool saturation and rate-limit or block sources opening many idle HTTP/2 connections.

Detection

  • Alert on HTTP/2 connections that remain open with a zero initial window size or otherwise idle beyond the configured timeout.
  • Track worker/thread pool exhaustion and sudden spikes in concurrent connections per source IP.
  • Baseline normal HTTP/2 connection lifetimes and flag long-lived connections with no request activity.
  • Correlate connection counts with server error rates and latency to spot slow-loris style resource exhaustion.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://httpd.apache.org/security/vulnerabilities_24.html Vendor Advisory
https://security.netapp.com/advisory/ntap-20231027-0011/ Third Party Advisory
https://httpd.apache.org/security/vulnerabilities_24.html Vendor Advisory
https://security.netapp.com/advisory/ntap-20231027-0011/ Third Party Advisory

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Source: NIST National Vulnerability Database (record CVE-2023-43622), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.